A hot melt adhesive is a thermoplastic material, solid at room temperature, which is applied in its molten form and will adhere to a surface when cooled to a temperature below its melting point. They differ from other liquid adhesives in that they set simply by cooling rather than by chemical curing or the evaporation of a solvent.
Typical hot melt adhesives comprise a backbone polymer or copolymer, a wax, and a tackifying resin. The most commonly used copolymer is ethylene vinyl acetate. The polymer or copolymer is used to impart flexibility and cold temperature resistance to the adhesive.
Tackifying resins are used to promote adhesion and improve the adhesive wet out of the substrates to be bonded. The resins typically used are solids, and are derived from rosin, natural terpene, or petroleum hydrocarbons. Due to their high Tg, solid tackifying resins are detrimental to both the flexibility and to the cold temperature resistance of the adhesive.
The wax is used to reduce the viscosity of the adhesive, increase the rate of set, and increase the heat resistance. Waxes are typically brittle, however, and are detrimental to flexibility and to the cold temperature resistance of the adhesive. Most commonly used waxes are paraffin waxes, microcrystalline waxes, and synthetic high melting point waxes. The latter class includes high density, low molecular weight polyethylene waxes, by-product polyethylene waxes, and Fischer-Tropsch waxes.
In recent years, substantial interest has grown in the bookbinding, publishing and printing industries, and in the graphic arts area generally with respect to the development of improved adhesive compositions which lend themselves, in book bindings and the like, to a quality referred to in the bookbinding industry as "lay flat." "Lay flat" refers to the ability of a hard cover book, paperback book, pocket book, booklet, manual, magazine, catalogue, directory such as a telephone directory, and pamphlet to mention only a few, to be opened and laid on a table top or other flat surface without springing closed or "mousetrapping" back to a closed position due to the rigidity and memory of the adhesive composition used in the backbone or binding of the book. Improved "lay flat" has been especially desirable in the computer industry where instruction manuals are used and hands must be free to manipulate computer keyboards while viewing the manual. Lay flat in also highly desirable for school textbooks.
U.S. Pat. No. 4,942,195 to Flanagan et al. teaches the use of a toughened hot melt adhesive comprising a radial styrene-butadiene block copolymer having a styrene content of greater than about 35% by weight, tackifying resins, and at least one wax diluent for bookbinding applications. This waxy diluent is either a paraffin wax or a microcrystalline wax. Flanagan contemplates the use of up to about 10% by weight of a synthetic high melting point wax such as a Fischer Tropsch wax but does not contemplate the use of more than 5% by weight of such a wax. Fischer Tropsch waxes have been known in the art to be detrimental to "lay flat" characteristics for bookbinding and are generally avoided for such bookbinding applications as one shot and two shot binding.
Reactive hot melt polyurethane adhesives(PUR's) or hot melt moisture cures(HMMC's) have been used to accomplish improved "lay flat" characteristics in books. PUR's depend on the presence of moisture to cure which involves crosslinking to form tough, resilient hot melts. The moisture is obtained from the atmosphere around the PUR and this curing process can take days or weeks. As a result, books are often shipped prior to cure and the books cannot, therefore, be tested for quality control prior to shipment. This may result in books that are not within specification. Cost is also a disadvantage with PUR's.
Surprisingly, the current inventors have found that a synthetic high melting point wax utilized along with a styrene-isoprene-styrene block copolymer, a benzoate plasticizer which is solid at room temperature, and a tackifying resin results in a fast setting hot melt with excellent peels resulting in excellent heat resistance, and excellent flexibility. Surprisingly, this hot melt adhesive exhibits exceptional "lay flat" characteristics for bookbinding applications while maintaining toughness.